Muscle Spindles: Special modified bundles of intrafusal muscle fibers, lying between muscle fibers, which can detect changes in muscle length and cause a sensory nerve impulse to be triggered on its nerve fiber carrying an impulse to the spinal cord and hence motor fibers that lead back to the muscle causing it to contract (the Myotatic Reflex).
Since fibers within the muscle spindle are referred to as intrafusal fibers, fibers outside the spindle, the muscle fibers, are called extrafusal.
Muscle spindles are proprioceptors which are part of the sensory feedback system that monitors information about body movement and position. This feedback is called proprioception.
A connective sheath surrounds the muscle spindles and attaches to the endomysium of the muscle fibers. The muscle spindle fibers are controlled by special motor neurons called gamma motor neurons or γ-motor neurons for short. Regular extrafusal muscle fibers are controlled by alpha-motor neurons (α-motor neurons).
Since it is attached to the muscle fibers, when the muscle fibers stretch, the muscle spindle fibers stretch as well and sensory nerves of the central part of the spindle fiber transmits information about the change in length which causes α-motor neurons in the spinal cord to trigger a reflexive contraction in the extrafusal fibers of the associated muscle, to prevent further change in length.
An example of how this reflexive action works is easily illustrated by placing your hand palm up in front of you with the arm bent 90 degrees at the elbow. If someone suddenly places a heavy weight in your hand, the hand will start to drop, causing the muscle fibers in the elbow flexors (biceps brachii, brachialis, etc.) to stretch.
The sensory nerves in the muscle spindles of those muscles then send action potentials to spinal cord which activates the α-motor neurons in the elbow flexor muscles. The muscles automatically contract and resist any further stretch, causing your hand to stop dropping without any conscious reaction on your part.
For more information see the Stretch Reflex to Passive or Static Stretching. Also see Physiology of Sport and Exercise by Jack H. Wilmore, David L. Costill, and W. Larry Kenney.
This page created 28 Sep 2012 13:17
Last updated 01 Mar 2016 18:54